“Highlights” calls attention to exciting advances in developmental biology that have recently been reported in Developmental Dynamics. Development is a broad field encompassing many important areas. To reflect this fact, the section spotlights significant discoveries that occur across the entire spectrum of developmental events and problems: from new experimental approaches, to novel interpretations of results, to noteworthy findings utilizing different developmental organisms.

Two halves make a whole (Axonal Regeneration Proceeds Through Specific Axonal Fusion in Transected C. elegans Neurons by Brent Neumann, Ken C.Q. Nguyen, David H. Hall, Adela Ben-Yakar, and Massimo A. Hilliard, Dev Dyn240:1365-1372) Two people in a relationship are said to become as one. However, after separation, their behaviors are not unlike that of injured axons: some wither and others continue along a new path. Here, Neumann et al. establish a new mode of axonal regeneration in Caenorhabditis elegans that may serve as a better model for the recently separated. Upon laser-based axonal transection, the proximal segment of an injured ALM or PLM neuron grows, circumnavigates the injury site, and reconnects with its distal fragment. By 24 hr, the repair is complete: plasma membranes have fused and anterograde and retrograde cytoplasmic diffusion is restored. If re-connection fails or is incomplete, the distal portion degenerates in a Wallerian manner. What's more, when two adjacent neurons are transected simultaneously (ALN and ALM or PLN and PLM), the proximal axon nearly always faithfully rejoins its distal end. The cues and signaling events that regulate re-connection and self-recognition remain unknown, but are poised for discovery within this genetically tractable animal model. Perhaps more of the recently separated should take note of the injured ALM and PLM, who after injury can regenerate a bond that is as strong as ever.

A little help from their friends (Neural Crest Cell Communication Involves an Exchange of Cytoplasmic Material Through Cellular Bridges Revealed by Photoconversion of KikGR by Mary Cathleen McKinney, Danny A. Stark, Jessica Teddy, and Paul M. Kulesa, Dev Dyn240:1391–1401) During the long pilgrimage from neural tube to target site, neural crest (NC) receive help along the way. McKinney et al. document cytoplasmic bridges that form between some neighboring, and dividing, migratory NC cells. Photoconversion of fluorescent reporter proteins within one of two bridged cells reveal that there is a bidirectional, yet unequal transfer of cytoplasmic contents. Model simulations based on in vivo measurements show that diffusion kinetics are consistent with active transport, suggesting cytoplasmic bridges are used for cell communication. Because cells re-orient their axes in the direction of the cytoplasmic bridge, and bridged cells are more likely to follow similar migratory pathways than non-bridged neighbors, the authors suspect this novel mode of NC communication may influence migratory behavior. The work suggests that NC help one another navigate to their journey's end.

Towing the line (Evidence for Partial Epithelial-to-Mesenchymal Transition (pEMT) and Recruitment of Motile Blastoderm Edge Cells During Avian Epiboly by Matt A. Futterman, Andrés J. García, and Evan A. Zamir, Dev Dyn240:1502–1511) When cells dive into a gastrulating embryo, or when neural crest cells trek across somites, do the cells collectively migrate together or do leading “free edge” cells pull their cohorts along? Futterman et al. address these questions in the little-studied avian epiboly model, where cells at the blastoderm edge migrate radially, causing the interior extraembryonic epithelium, or Area Opaca (AO), to expand several hundred-fold until it envelopes the yolk. They find that, unique from inner zone cells, edge zone cells undergo a partial epithelial to mesenchymal transition (pEMT). The cells have dual characteristics, strongly expressing the mesenchymal marker vimentin (VM), and also the epithelial markers cytokeratin, β-catenin, and E-cadherin. Supporting the towing model, VM filaments at the edge extend several microns, a sign that they are under considerable tension. As the blastoderm periphery expands, where do new edge zone cells come from? Bromodeoxyuridine (BRDU) pulse-chase experiments suggest that edge zone cells do not proliferate, but rather inner zone cells are recruited to the edge. Together, these findings raise new questions about what are the molecular and biophysical mechanisms that propel cells into a pEMT state. With further investigation, the rediscovered avian epiboly model may drag to the limelight new insights into multicellular migration.